Heating apparatus for electrophotographic copiers

ABSTRACT

Radiant energy heater for fusing a toner powder image onto the surface of an image bearing support member in electrophotographic copiers. The heater comprises a housing which supports both a conventional active radiant energy heating element and a passive radiant energy heating element such as a high heat capacity insulator material. When the copier is in a stand-by mode, the housing is closed and the active heating element is energized to maintain it and the insulator material in a fusing condition. To fuse an image, the housing is opened to direct heat to an image bearing medium transported thereacross from both the active heating element and the insulator. In this way, the insulator provides a passive source of radiant energy to initially supplement the active heating element until it is able to supply a sufficient amount of uniform heat energy to effectively fuse the image alone. The system thus allows the first developed electrophotographic powder image of a copying operation to be transported beneath the heater immediately while, at the same time, avoiding the need for excessive amounts of power that would otherwise be required to bring the active heating elements to a stable heating condition rapidly.

United States Patent [191 Moore Nov. 25, 1975 1 HEATING APPARATUS FOR ELECTROPHOTOGRAPHIC COPIERS [75] Inventor: Robert A. Moore, Amherst, NH.

[73] Assignee: ltek Corporation, Lexington, Mass.

[22] Filed: Feb. 19, 1974 [21] Appl. No: 443,286

[52] US. Cl 219/216; 219/341 [5] lnt. Cl. HOSB 1/00 [58] Field of Search... 219/216, 388, 341, 347-349, 219/350; 355/9; 250/317-319; 118/637;

[56] References Cited UNITED STATES PATENTS 3,356.831 12/1967 Andrus et a1. 219/216 X 3,449,546 6/1969 Dhoble 219/216 3,622,745 11/1971 Shepard 219/388 X 3,811,828 5/1974 Ohta 219/216 X Primary Examiner-C. L. Albritton Attorney, Agent, or Firm--Homer 0. Blair; Robert L. Nathans; Gerald H. Glanzman [57] ABSTRACT Radiant energy heater for fusing a toner powder image onto the surface of an image bearing support member in electrophotographic copiers. The heater comprises a housing which supports both a conventional active radiant energy heating element and a passive radiant energy heating element such as a high heat capacity insulator material. When the copier is in a stand-by mode, the housing is closed and the active heating element is energized to maintain it and the insulator material in a fusing condition. To fuse an image, the housing is opened to direct heat to an image bearing medium transported thereacross from both the active heating element and the insulator. In this way, the insulator provides a passive source of radiant energy to initially supplement the active heating element until it is able to supply a SUffiClCl'lI amount of uniform heat energy to effectively fuse the image alone. The system thus allows the first developed electrophotographic powder image of a copying operation to'be trans ported beneath the heater immediately while at the same time, avoiding the need for excessive amounts of power that would otherwise be required to bring the active heating elements to a stable heating condition rapidly.

12 Claims, 2 Drawing Figures HEATING APPARATUS FOR ELECTROPI-IOTOGRAPHIC COPIERS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to radiant energy heating apparatus, and more particularly to an improved radiant energy heating apparatus for fusing toner powder images onto the surface of an image hearing medium in electrophotographic copiers.

2. Description of the Prior Art An important goal in the development of modern electrophotographic copiers is to provide finished copies of original documents in as short a period of time as possible. In attempting to reach this goal, one area of the copier that has received substantial attention is the fusing station through which the developed electrophotographic toner powder images are transported to fuse or fix the powder images to the surface of the support member on which they are carried. Although a variety of fusing systems have been developed and are being used in the art, for example, systems utilizing vapor fusing or contact fusing techniques, those systems that fuse by radiant energy appear to offer the most advantages with regard to permitting effective high speed copying. Specifically, radiant energy systems usually have a somewhat faster warm-up time, are less complex in design, and, in general, are more efficient in directing the generated heat energy to the images to be fused.

Those radiant energy fusing systems that are available in the prior art, however, are still not fully satisfactory for several reasons.

One of the main problems is that once the heating element is energized, it requires a certain period of time before it reaches an equilibrium condition and begins radiating sufficient amounts of heat in a stable manner. This is because in the initial stages after the heating elements are energized, much of the heat generated is used to heat up the envelope of the lamp or the air around the lamp, and as much as a minute or so is needed before sufficient amounts of heat can be uniformly applied to the powder image to permit fusing to be carried out.

To overcome this deficiency, a variety of corrective measures have been developed. For example, in some systems, a plurality of spaced heating lamps are provided so as to increase the effective size of the fusing station, and, in this way, cause the powder images to be heated for a longer period of time. The problem with such designs, however, is that it is also often necessary to move the images through the fusing station quite slowly to insure that effective fusing will take place. Also, since there are so many heating lamps, substantial amounts of power are required to drive them and this can result in a substantial waste of electricity as well as in the generation of substantial amounts of heat within the machine which can cause problems.

Other types of systems attempt to solve this stabilization problem by maintaining the heating elements energized in a stand-by mode when the machine is not in use. With this technique, there is also a substantial amount of wasted heat and energy. Also, since the heating elements are continuously giving off heat, there is the danger that the image bearing support medium, which is normally stationary when copies are not being made, will be charred or burned. Because of this problem, it is usually necessary to maintain the heating ele- 2 ments somewhat below the fusing temperature and to then rapidly raise them to the fusing temperature when copying is desired, to keep the belt moving, to retract the heaters, or the like which increases the complexity of the system.

Yet, another type of fusing system attempts to overcome the dangers of burning or charring the support member by enclosing the radiant energy sources within a suitable housing so that it may be maintained at the fusing temperature when in a stand-by mode. In such systems, when copying is to be carried out, the housing is opened so that the radiant energy will be more directly applied to the powder image to be fused. With such systems there is still the problem that the air surrounding the housing is at a significantly lower temper ature than the interior of the housing and as a result there will still be instabilities in the amount of radiant energy applied to the image until the surrounding air itself becomes uniformly heated. Thus, it is still necessary to utilize significant amounts of power to enable rapid fusing of a first copy.

In general, there is no system presently available which can effectively permit very rapid fusing of the I first developed electrophotographic powder image in a copying operation while, at the same time, avoid the need for excessive amounts of power that necessitates special power sources and generally makes the machine more expensive to operate.

SUMMARY OF THE PREFERRED EMBODIMENT OF THE INVENTION In accordance with the present invention, a novel radiant energy fusing system has been provided which significantly overcomes many of the above described inadequacies of the prior art by permitting essentially immediate fusing of powder images without requiring excessive amounts of power.

In accordance with a presently preferred embodiment of the invention, these improved characteristics are obtained by providing a fusing system that includes the conventional active radiant energy heating elements together with a suitable passive heating element capable of supplementing the active heating elements during the period in which the heat they radiate are approaching a stable equilibrium condition. Specifically, the fusing system comprises an active radiant energy source such as an infrared lamp or the like supported within a housing which may be selectively opened and closed. A portion of the housing is also substantially filled with a passive heat source such as a suitable insulator material capable of storing large amounts of heat and then releasing that heat in the form of radiation when opened to the atmosphere.

The system operates as follows: When in a stand-by mode, the housing is closed and the active heat source is energized to bring it and the insulator material to the fusing condition. When it is then desired to fuse an image, the housing is opened so as to direct radiant energy to the powder image to be fused from both the lamp and the insulator material. Together, the active and passive heat sources will supply sufficient heat to permit immediate fusing of the powder image carried thereacross notwithstanding the fact that the active source itself may not immediately radiate sufficient amounts of stable heat for fusing. The passive heat source will supply substantial amounts of heat for a period of from 30 seconds to one minute before it cools down but, by then, the active heat source will have 3 been gradually brought into equilibrium and be ade quate along for fusing.

Thus, with the present invention, images can be fused almost immediately without using excessive amounts of power that would otherwise be necessary to rapidly bring the active heating lamps to equilibrium. As a result, the system can be plugged into conventional wall outlets rather than requiring special power sources and this, besides saving energy, provides the significant advantage of rendering the copying machine more easily portable.

Although the heat lamps are kept energized in a standby mode, they are enclosed within a well insulated housing so that there is not too much energy loss to the atmosphere. For the same reason. there is no danger of the image support material being burned or charred during this period.

In general, the present invention is relatively simple in construction yet is highly effective in permitting rapid copying with minimum power. Yet further advantages and features of the invention will be set out hereinafter in conjunction with the detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 somewhat schematically illustrates a plan view of the fusing station in accordance with a preferred embodiment of the present invention while in the stand-by mode.

FIG. 2 illustrates, in schematic cross-sectional form, the fusing station of FIG. 1 in the operating mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 illustrate, in somewhat schematic form, a fusing station according to a presently preferred embodiment of the invention for use in conjunction with electrophotographic copying machines. The fusing station, generally designated by reference number I0, is supported by suitable structure (not shown for clarity) adjacent a transport system generally identified by reference number 11 which is provided to carry a powder image bearing support member across the fusing station for fusing the powder image onto the surface thereof. Transport system 11 may comprise a roller driven belt 12 or some other conventional transport mechanism for carrying a powder image bearing sheet of paper or other medium to the fusing station from earlier stations in the copying machine as understood by those skilled in the art to fuse and fix the image thereto. Alternatively, as shown in the FIGS., the belt 12 may itself comprise the image bearing medium upon which the toner powder image, schematically illustrated at 26, is to be fused for later transfer to a final substrate such as paper as is described in copending US. patent application Ser. No. 403,696 to William R. Buchan and Robert A. Moore filed on Oct. 3, 1973, and entitled ELECTROPl-IOTOGRAPHIC TONER TRANSFER AND FUSING APPARATUS AND METHOD. It should also be emphasized at this point that the term "fusing" as used in the present application is intended to refer to the act of heating the powder image to a condition where the powder particles coalesce and become flowable whether or not it is actually fixed to the image bearing support medium or carried thereon to a later stage of the copier to be transferred therefrom to a sheet of paper or the like.

4 Fusing station comprises an elongated generally rectangular shaped housing I3 that extends across the width of belt 12, as more clearly seen in FIG. 1 and that includes a main body portion 14 and a lid portion I5 5 joined together by hinge 16. As shown in FIG. 2, housing 13 has an outer shell I8, 19, defining the main body and lid portions, respectfully, made of stainless steel although other materials capable of withstanding the high temperatures encountered could also be used.

Supported within the main body portion 14 of housing 13 are two sources of radiant energy 31 and 32 (FIG. 2). Sources 31 and 32 extend across the length of the housing and preferably comprise quartz-tungsten lamps which are efficient sources of thermal energy and have a relatively long operating life although it should be understood that other heat sources as well as different numbers of sources may also be employed if desired. Lamps 31 and 32 are coupled by appropriate circuitry to an external power source (not shown) of conventional type as well as to the usual controls incorporated in the copying machine for turning them on and off at the appropriate time.

The shell 19 forming the lid portion 15 of the housing is substantially filled with a suitable insulator material 21 capable of storing and later radiating heat energy supplied to it by lamps 31 and 32 as will be explained hereinafter. In particular, material 21 is one which has the properties of low thermal conductivity, high thermal capacity and high thermal emissivity, and one material that has been found suitable for this application is a ceramic fiber available from Babcock-Wileox Corp. under the name Kaowool". As shown in FIG. 2, the interior of shell 19 defining the body portion 14 of housing 13 is also substantially covered with similar insulating material 22 to minimize energy loss through the walls of the housing.

FIG. 1 shows housing 13 in a closed or stand-by condition while FIG. 2 illustrates the same housing in an open condition for fusing. Thus, appropriate mechanical structure is included in the fusing station 10 to effect the necessary opening and closing of the housing 13. This structure can take many different forms and, therefore, is only schematically illustrated in the FIGS. Specifically, as illustrated, body portion 14 is coupled to a suitable support in the machine by a hinge coupling 23 which extends substantially across the length of the housing so that the housing may freely pivot around it relative to the support. Also, a suitable'linkage 24 coupled to a suitable push-pull mechanism is hingedly coupled to lid portion 16 as illustrated at 27. From the FIGS., it should be evident that when linkage 24 is pulled, the housing will open around hinge l6 and rotate to face downwardly around hinge 23 until it reaches the position shown in FIG. 2. In a similar manner, by applying a pushing force on linkage 24, or by some other conventional mechanism, the housing can also be returned to the FIG. 1 condition as desired.

The manner in which the fusing station 10 is effective in immediately fusing a toner powder image carried thereacross on a support medium 12 will now be described in conjunction with the copying machine with which the fusing station is associated. Initially, when the machine is totally shut down, for example overnight, the housing 13 will be in the condition shown in FIG. I with no power being supplied to lamps 31 and 32. When the machine is turned on in the morning, for example, preparatory to a day's use, the following will occur. Bulbs 31 and 32 will be energized while housing 13 will remain in the closed condition of FIG. 1. During this warm'up period. therefore, the interior ofthe housing will gradually be warmed up to the fusing condition. Due to the fact that the housing is well-insulated, there will not be much energy loss through the shells 18, 19 and the interior of the housing will reach the necessary fusing temperature of, for example, between 600 to 1000C. within the relatively short period of time of about lk minutes. Furthermore, because the housing is enclosed and well insulated, this relatively short warmup period can be accomplished without applying excessive amounts of power to the bulbs as is generally required for rapid warm-up in prior art systems. As the housing warms up, the insulator material 21 will, due to its high heat capacity, store a significant amount of the heat energy applied to it.

When the bulbs 31 and 32 and the interior of the housing in general has reached the desired fusing condition, the applied power can then generally be reduced to a level sufficient to maintain the housing at the proper fusing temperature (appropriate sensors will be included in the system for monitoring purposes) and it will remain in this stand-by mode until it is desired to make a copy.

When now it is desired to reproduce a document, the machine is actuated to initiate the copying process and a toner powder image 26 will ultimately be formed on support 12 in a manner understood by those skilled in the art, and, thereafter will be carried in the direction indicated by arrow 28 to fusing station to heat the toner image on support 12. Immediately before the image 26 reaches fusing station 10, the linkage 24 will be automatically actuated to open the housing into the position shown in FIG. 2 which may be termed the operating or fusing mode of the system. Inasmuch as the lamps have been preheated, they will immediately be radiating substantial amounts of heat. However, the opening of the housing will also bring the lamps into sudden contact with the relatively cool air around the housing, and, as a result, substantial amounts of the heat from the lamps will initially be used to heat up the surrounding air and will not supply very uniform amounts of heat to image 26. Accordingly, to insure proper fusing of the image, it would normally be necessary to wait until the surrounding air has become heated and the radiation reaching the image becomes stable and uniform and this can take from 30 to 60 seconds unless very large amounts of power are applied to the lamps.

This waiting period before the first copy can be fused has been substantially eliminated by the present invention without requiring large amounts of power by the use of insulator material 21. Specifically, as shown in FIG. 2, when the housing is opened, lid portion will be positioned in advance of bulbs 31 and 32 relative to incoming image 26. Also, as mentioned above, this lid is filled with insulator material 21 which has stored up large amounts of heat while the housing was in the stand-by mode. Accordingly, when housing 13 is opened, material 21 will begin radiating substantial amounts of heat toward the incoming image, and thus assist in the fusing operation while lamps 31 and 32 are stabilizing.

In other words, during the period that bulbs 31 and 32 (which may be considered active radiation sources since they are driven by an external power source) are approaching an equilibrium condition, the material 21 in lid 15 (which may be considered a passive radiation source since it is not externally powered) will provide sufficient heat to permit image 26 to be immediately melted on support 12. Put yet another way, the effect of passive radiation source 21 is to supplement the active sources 31 and 32 by enlarging the effective size of the fusing station during the period when active sources 31 and 32 are unstable so that image 26 can immediately be carried through the fusing station 10 at the normal speed. Insulator 21 will gradually cool off but, nonetheless, will supply enough heat to assist bulbs 31 and 32 for the required period of about 30 to 60 seconds, at which time they will reach equilibrium and be able to effectively fuse image 26 without assistance from passive source 21.

As long as a copying operation is being carried out, housing 13 will remain in the FIG. 2 position for fusing. When the operation is completed, housing 13 will automatically return to the FIG. 1 condition to reheat insulator material 21 for use in the next copying operation.

In an operative embodiment of the system for fusing a toner image to a standard 8% inch wide document, container 13 is approximately 10 inches long to insure that the entire image will be fused. Lid portion 15 and body portion 14 are about 3--4 inches wide, the body portion is about 2 inches thick and the lid is about one inch thick and substantially filled with insulator material 21. With such dimensions, image bearing medium 12 may be carried across the open housing at a belt speed of about 10 inches per second and be spaced from the housing by a distance of about VB A inch to insure that the housing or insulator material will not contact or disturb the powder image on the belt. Housing 13 can be fully opened in about lk to 2 seconds and this is essentially the only waiting period necessary for first copy fusing once the initial warm up has been completed as above described.

Perhaps the most important feature of the present in' vention is not only that it is possible to obtain rapid first copy speeds, but that it can do so without utilizing large amounts of power. This is because, for initial warm up, the bulbs are enclosed within a well insulated housing and thus can be brought to the fusing temperature relatively quickly (ll to 2 minutes) without large amounts of input power, and, also for fusing, the passive radiator 21 will supply sufficient amounts of heat to enable immediate fusing while bulbs 31 and 32 are gradually brought to an equilibrium condition. Also, since high first copy speed can be accomplished with reduced power, bulbs 31 and 32 will tend to have a longer operating life and this makes the machine more economical. Finally, because the housing 13 is closed when in the stand-by mode, it is possible to maintain its interior at the fusing temperature without any danger of charring or burning support 12 which is stationary during that period, and in general, without wasting a lot of energy.

While what has been described above is the presently preferred embodiment of the invention, it should be recognized that it could also take a variety of other forms. For example, the housing 13 could be opened and closed in many different ways than the structure schematically illustrated and could be designed in other than the clamshell" construction shown. Furthermore, if desired, it is possible to include an additional preheating system in the housing, such as a coil or the like, to heat up the insulator material in place of or in addition to bulbs 31 and 32.

Because many additions, alterations, or omissions may be made from the present invention without departing from the spirit thereof; it should be understood that the invention should be limited only as required by the scope of the following claims.

What is claimed is:

1. ln an electrophotographic copying system, heating apparatus for fusing a powder image on the surface of an image bearing support member carried thereacross, said heating apparatus comprising:

a. an active radiant energy heat source;

b. a passive radiant energy heat source, said passive radiant energy heat source comprising means for storing a substantial amount of heat energy applied thereto;

c. means for applying heat energy to said passive radiant energy heat source to be stored therein; and,

d. means for selectively releasing the heat energy stored within said passive radiant energy heat source and for directing radiant energy therefrom to said powder image for supplementing said active radiant energy heat source during periods when said active radiant energy heat source may supply insufficient heat to said powder image to effectively fuse it to the surface of said support member.

2. Apparatus as recited in claim 1 wherein said means for releasing the heat energy stored within said passive radiant energy heat source comprises:

a. housing means for supporting said passive radiant energy heat source, said housing having a closed position and an open fusing position; and,

b. means for opening said housing to said open fusing position for directing radiant energy from said passive radiant energy heat source to said powder image.

3. Apparatus as recited in claim 2 wherein said housing means comprises means for supporting both said active radiant energy heat source and said passive radiant energy heat source, and wherein said means for applying heat energy to said passive radiant energy heat source comprises means for directing radiant energy thereto from said active radiant energy heat source when said housing is in said closed position.

4. In an electrophotographic copying system. heating apparatus for fusing a powder image on the surface of an image bearing support member carried thereacross, said heating apparatus comprising:

a. an active radiant energy heat source;

b. a passive radiant energy heat source, said passive radiant heat source comprising means for storing a substantial amount of heat energy applied thereto and for radiating said heat energy stored therein to said powder image for supplementing said active radiant energy heat source during periods when said active radiant energy heat source supplies insufficient heat to said powder image for effectively fusing it on the surface of said support member;

c. a housing having an open fusing position and a closed stand-by position;

d. means for supporting said active radiant energy heat source and said passive radiant energy heat source within said housing; and,

e. means for opening said housing to said fusing position for directing radiant energy to said powder image bearing support member from both said active and passive radiant energy heat sources.

5. Apparatus as recited in claim 4 wherein said passive heat source comprises an insulator material having a high thermal capacity for storing large amounts of heat energy.

6. Apparatus as recited in claim 5 wherein said housing includes means for positioning said insulator material in advance of said active radiant energy heat source relative to said powder image bearing support member carried thereacross when said housing is in said open fusing condition.

7. Apparatus as recited in claim 5 and further including means for applying heat energy to said insulator material for storing said heat energy therein when said housing is in said closed stand-by position.

8. Apparatus as recited in claim 7 wherein said means for applying heat energy to said insulator material comprises means for energizing said active radiant energy heat source when said housing is in the closed stand-by position for directing heat to said insulator material from said active radiant energy heat source.

9. Apparatus as recited in claim 8 wherein said active radiant energy heat source comprises a quartz-tungsten lamp.

10. In an electrophotographic copying system, heating apparatus for fusing a powder image on the surface of an image bearing support member carried thereacross, said heating apparatus comprising:

a. a housing having a closed stand-by position and an open fusing position;

b. an active heat source supported within said housc. a passive heat source supported within said housing; and,

d. means for opening said housing to said fusing position for directing radiant energy from both said active and passive heat sources to a powder image bearing support member carried thereacross, said passive heat source supplementing said active heat source during periods when said active heat source supplies insufficient heat to said powder image for effectively fusing it on the surface of said support member.

11. Apparatus as recited in claim 10 and further including means for heating said passive heat source when said housing is in said closed position.

12. Apparatus as recited in claim 4 wherein said material comprises an insulator material.

# i i I 

1. In an electrophotographic copying system, heating apparatus for fusing a powder image on the surface of an image bearing support member carried thereacross, said heating apparatus comprising: a. an active radiant energy heat source; b. a passive radiant energy heat source, said passive radiant energy heat source comprising means for storing a substantial amount of heat energy applied thereto; c. means for applying heat energy to said passive radiant energy heat source to be stored therein; and, d. means for selectively releasing the heat energy stored within said passive radiant energy heat source and for directing radiant energy therefrom to said powder image for supplementing said active radiant energy heat source during periods when said active radiant energy heat source may supply insufficient heat to said powder image to effectively fuse it to the surface of said support member.
 2. Apparatus as recited in claim 1 wherein said means for releasing the heat energy stored within said passive radiant energy heat souRce comprises: a. housing means for supporting said passive radiant energy heat source, said housing having a closed position and an open fusing position; and, b. means for opening said housing to said open fusing position for directing radiant energy from said passive radiant energy heat source to said powder image.
 3. Apparatus as recited in claim 2 wherein said housing means comprises means for supporting both said active radiant energy heat source and said passive radiant energy heat source, and wherein said means for applying heat energy to said passive radiant energy heat source comprises means for directing radiant energy thereto from said active radiant energy heat source when said housing is in said closed position.
 4. In an electrophotographic copying system, heating apparatus for fusing a powder image on the surface of an image bearing support member carried thereacross, said heating apparatus comprising: a. an active radiant energy heat source; b. a passive radiant energy heat source, said passive radiant heat source comprising means for storing a substantial amount of heat energy applied thereto and for radiating said heat energy stored therein to said powder image for supplementing said active radiant energy heat source during periods when said active radiant energy heat source supplies insufficient heat to said powder image for effectively fusing it on the surface of said support member; c. a housing having an open fusing position and a closed stand-by position; d. means for supporting said active radiant energy heat source and said passive radiant energy heat source within said housing; and, e. means for opening said housing to said fusing position for directing radiant energy to said powder image bearing support member from both said active and passive radiant energy heat sources.
 5. Apparatus as recited in claim 4 wherein said passive heat source comprises an insulator material having a high thermal capacity for storing large amounts of heat energy.
 6. Apparatus as recited in claim 5 wherein said housing includes means for positioning said insulator material in advance of said active radiant energy heat source relative to said powder image bearing support member carried thereacross when said housing is in said open fusing condition.
 7. Apparatus as recited in claim 5 and further including means for applying heat energy to said insulator material for storing said heat energy therein when said housing is in said closed stand-by position.
 8. Apparatus as recited in claim 7 wherein said means for applying heat energy to said insulator material comprises means for energizing said active radiant energy heat source when said housing is in the closed stand-by position for directing heat to said insulator material from said active radiant energy heat source.
 9. Apparatus as recited in claim 8 wherein said active radiant energy heat source comprises a quartz-tungsten lamp.
 10. In an electrophotographic copying system, heating apparatus for fusing a powder image on the surface of an image bearing support member carried thereacross, said heating apparatus comprising: a. a housing having a closed stand-by position and an open fusing position; b. an active heat source supported within said housing; c. a passive heat source supported within said housing; and, d. means for opening said housing to said fusing position for directing radiant energy from both said active and passive heat sources to a powder image bearing support member carried thereacross, said passive heat source supplementing said active heat source during periods when said active heat source supplies insufficient heat to said powder image for effectively fusing it on the surface of said support member.
 11. Apparatus as recited in claim 10 and further including means for heating said passive heat source when said housing is in said closed position.
 12. Apparatus as recited in claim 4 wherein said material comprises an insulator material. 